Plasma immersion ion implantation has emerged as a highly productive alternative to ion beam implantation. A workpiece such as a semiconductor wafer is immersed in a plasma formed from a gas containing a chemical species to be ion implanted, such as a dopant. For dopant species used in semiconductors, the process gas may be a hydride or fluoride of a dopant species such as arsenic, boron or phosphorus. The ion implant dose rate is a function of the plasma ion density, while the ion implant depth profile is a function of the bias voltage applied to the workpiece. As used herein the term ion implant dose refers to the concentration of ion implanted atoms in the implanted layer of the workpiece, the concentration typically being measured in atoms per cubic centimeter. The term dose rate refers to the time of increase of the dose during ion implantation. In some applications, the ion implantation is performed in accordance with a predetermined pattern on the workpiece surface established by a photoresist mask deposited by a photolithographic process. In this case, the photoresist mask is a film thicker than the penetration depth of the ions. For many applications such as doping of the source and drain of a field effect transistor, the junction depth is determined by the implant depth profile, which is controlled by the bias voltage applied to the workpiece. For typical junction depths, the bias voltage is relatively high, in the range of tens of kiloVolts. The impact of the ions within the photoresist layer damages the photoresist by breaking the hydrogen-carbon bonds in the photoresist. The photoresist is sufficiently porous to permit the free hydrogen to outgas through the photoresist. The damage leads to carbonization of the photoresist film, by the loss of hydrogen atoms and, ultimately, formation of carbon-carbon bonds, starting at the top surface of the photoresist layer. As the carbon-carbon bonds proliferate, a diamond-like non-porous region begins to grow in the photoresist film, starting at the top surface of the photoresist film, and progressing downward. Once this region has grown to a sufficient thickness, it traps hydrogen gas produced in the lower (remaining) portion of the photoresist film that has not yet carbonized. The trapped hydrogen gas forms bubbles in the photoresist film that eventually break, causing the photoresist film to fail, generating contamination at unacceptable levels. Another problem is that the diamond-like carbonized photoresist is difficult to remove after completion of the plasma immersion ion implantation process. This is particularly true if the entire photoresist layer has been carbonized upon completion of the ion implantation process. Removal of all the photoresist after completion of the ion implantation process is required to avoid process contamination in subsequent fabrication steps.
The problem of removing the carbonized photoresist has been addressed by employing a three-step photoresist ashing process to remove all of the photoresist in a single continuous effort, consisting of (1) a de-ionized water flush of the wafer surface, (2) an oxygen ashing step, and (3) exposure to sulfuric acid and hydrogen peroxide. This three-step photoresist removal process has three limitations. First, it can neither prevent nor cure the contamination caused by bubbling or failure of the photoresist. Second, it does not necessarily remove all of the carbonized photoresist. Third, it is believed that the oxygen chemistry of the second step of the ashing process may remove implanted dopant species such as boron from some areas of the workpiece, degrading uniformity of dopant distribution across the workpiece surface.
What is needed is a plasma immersion ion implantation process that does not cause the photoresist layer to fail while permitting all of the photoresist material to be removed after completion of the implantation process.